1. Field of the Invention
This invention relates in general to the field of spectral shift pressurized water nuclear reactors and in particular to a gas displacement spectral shift pressurized water nuclear reactor.
2. Description of the Prior Art
In typical pressurized water nuclear reactors, control over the fission process, or reactivity control is accomplished during reactor operation by varying the amount of neutron moderating and absorbing materials within the core of the reactor. One method to effectuate reactivity control is by the use of control rods containing such neutron absorbing materials or poisons which are inserted within the reactor core. Control over the fission process may be accomplished by varying the number of control rods, the size of the control rods and their radial and axial locations within the core. Burnable poisons (by the fissioning process) and poisons dissolved in the reactor coolant can additionally be used for purposes of such control.
In order to lengthen the core life, (time between refuelings), it is typical, in conventionally designed commercial pressurized water reactors, to design in an excess of reactivity at reactor start-up. The excess reactivity is controlled as stated above, and is gradually depleted over the extended life of the core. Soluble boron, dissolved in the reactor coolant is most often used in light water PWR's to control the initial excess reactivity. As the excess reactivity in the core is depleted during the reactor operation, the neutron absorbing boron is gradually removed so as to utilize the original excess reactivity to maintain the fission process. While this control arrangement provides an effective means of controlling a nuclear reactor over an extended core life, the neutron absorbing boron used during core life absorbs neutrons and removes reactivity from the reactor core that could otherwise be used in a more productive manner. For example, the reactivity could be used to convert fertile material to plutonium or to fissile uranium which even further extends the reactor core life by fissioning the then generated fissile material. Without such conversion, however, the consumption of reactivity is an inefficient depletion of uranium resulting in higher fuel costs than would otherwise result. In view of the above, it would be an obvious advantage to be able to extend the life of a core having an initial amount of excess reactivity while not suppressing the excess reactivity with neutron absorbing materials, but rather using the excess reactivity in a positive manner thereby providing an extended core life with a significantly lower overall fuel cost.
It is well known that fuel element enrichment can be reduced and the conversion ratio of producing fissile materials can be increased by employing a "hardened" (higher neutron energy) spectrum during the first part of the fuel cycle to reduce excessive reactivity and to increase the conversion of fertile material to fissile material; then employing a "softer" (lower energy) neutron spectrum during the latter part of the fuel cycle to increase reactivity and extend the core life by fissioning the previously generated fissile material. One such method utilizing the above is known as spectral shift control which provides a reactor with an extended core life while reducing the amount of neutron absorbing material in the reactor core. One example of such method of control comprises a mechanical spectral shift reactor whereby hollow displacer rods are provided within fuel assemblies within the core (which, of course, displace an equal volume of water within the fuel assemblies) and which are mechanically withdrawn or punctured to accomplish water flooding of the available volume. In the early stages of core life, the neutron spectrum is hardened by the displacement of a portion of the water within the core by the displacer rods. The spectrum is later softened by the addition of water within the core by the aforesaid rod withdrawal or puncturing. Patent Application Ser. No. 217,054, now U.S. Pat. No. 4,432,930, entitled "Spectral Shift Reactor Control Method" by A. J. Impink, Jr. et al., filed on Dec. 16, 1980, assigned to Westinghouse Electric Corporation, discloses one such mechanical shift reactor.
Another method of achieving a spectral shift is to utilize heavy water or deuterium oxide to replace an equivalent volume of core water during the early stages of core life than to gradually reduce the volume of heavy water and replace it with regular reactor water during the later stages of core life. The less effective moderator, heavy water, allows for less fuel enrichment and a higher ratio of converting fertile material to fissile which in combination provides for a reduction of fuel costs and an extension of core life. An example of this art is found in Patent Application Ser. No. 626,847 entitled "Fuel Assembly" by R. K. Gjertsen et al., filed on July 2, 1984, and assigned to Westinghouse Electric Corporation.
While the concept of spectral shift control of a pressurized water nuclear reactor exists in various forms, there exists a continuing need to more effectively implement the concept by providing apparatus and methods having decreased complexity, lower overall costs, compatability with existing commercially available nuclear reactors, and which, of course, further enhance the safety of nuclear reactors.
It is, therefore, a primary object of the present invention to provide apparatus and a method for a pressurized water nuclear reactor which includes means for varying the reactor coolant water volume over the life of a core by replacing a portion of the water with a gas having negligible neutron absorption and moderating properties during the early stages of core life and then gradually replacing the gas with ordinary reactor coolant as the core life decreases.